Seed Production Environment, Time of Harvest, and the Potential Longevity of Seeds of Three Cultivars of Rice (Oryza sativa L.)

Transcription

1 Annals of Botany 72: , 1993 Seed Production Environment, Time of Harvest, and the Potential Longevity of Seeds of Three Cultivars of Rice (Oryza sativa L.) R. H. ELLIS, T. D. HONG and M. T. JACKSON* Department of Agriculture, University of Reading, Earley Gate, P.O. Box 236, Reading RG6 2AT, UK and * Genetic Resources Center, International Rice Research Institute, P.O. Box 933, 1099 Manila, Philippines Accepted: 14 June 1993 Changes in seed quality (assessed by potential longevity, i.e. the value of the seed lot constant X; of the seed viability equation) in three contrasting cultivars of rice (Oryza sativa L.) were monitored during seed development and maturation in two temperature regimes, viz 28/20 C and 32/24 C (12/12 h), provided by controlled environments. Mass maturity (defined as the end of the seed-filling phase) varied only between 18 and 20 dafter 50 % anthesis. In five of the six treatment combinations maximum potential longevity was not achieved until d after mass maturity. In contrast, the maximum potential longevity of seeds of a japonica rice cultivar produced in the warmer regime was obtained in the first harvest after mass maturity. After mass maturity, the potential longevity of the japonica rice seed lots produced in the warmer environment was much less than that for the cooler environments. Maximum potential longevity was also consistently greater in the cooler than the warmer regime for the two indica cultivars, although the difference in K, was small ( ). The deleterious effect of increase in temperature on seed quality development was not detected until after mass maturity. Maximum potential longevity in the cooler regime was greatest in the glutinous indica cultivar (K, = 3.9) and least in the japonica cultivar (K, = 3.1). It is concluded that the japonica cultivar is not as well adapted to warm seed production regimes as the indica cultivars. Consequently, subject to confirmation, this research suggests that the seed production of japonica cultivars for long-term genetic conservation should be undertaken, whenever possible, in warm temperate environments. Key words: Oryza sativa L., rice, genebanks, seed development, seed storage, seed longevity, temperature. INTRODUCTION There is now considerable evidence in several cereals (Kameswara Rao et al., 1991; Pieta Filho and Ellis, 1991 a; Ellis and Pieta Filho, 1992) and also other crops (Demir and Ellis, 1992a, b, 1993; Zanakis, Ellis and Summerfield, 1994) that developing and maturing seeds do not attain maximum ability to survive air-dry storage until some time after mass maturity-defined as the end of the seedfilling phase (Ellis and Pieta Filho, 1992). Several other facets of seed quality have also been shown to continue to improve during the natural seed maturation that occurs subsequent to mass maturity; for example, field emergence ability in barley (Hordeum vulgare L.) (Pieta Filho and Ellis, 1991 b), and seedling growth potential in pepper (Capsicum annuum L.) (Demir and Ellis, 1992 b). These results contradict the general hypothesis of Harrington (1972) that seed quality reaches maximum values at the end of the seedfilling phase and that thereafter viability and vigour decline-even though at least the first part of this hypothesis is supported by some recent investigations (for example, TeKrony, Egli and Phillips, 1980; Rasyad, Van Sanford and TeKrony, 1990). In recent investigations on the lower limit to the negative logarithmic relation between seed.longevity and moisture content in air-dry storage in three ecogeographic races of rice (Oryza sativa L.) it was found that japonica cultivars had poorer longevity characteristics than those of either /93/ $08.00/0 indica or javanica cultivars (Ellis, Hong and Roberts, 1992)-as Chang (1991) had suggested earlier. In that work, seed storage longevity was assessed in two ways. First by potential longevity as defined by the value of the seed lot constant Kj in the following equation which describes the seed survival curve in anyone (constant) air-dry environment: v=kj-p/o" (1) where v is probit percentage viability after p days in storage and 0" is the standard deviation of the frequency distribution of seed deaths in time (d). Second, by comparing the negative logarithmic relations between seed longevity (0") and moisture content (m, % wet basis) at one temperature: 10glO 0" = K -Cw loglo m (2) where K is an intercept constant and Cw is the sensitivity of longevity to moisture content (Ellis and Roberts, 1980a). Both Kj and K were poorer for japonica compared with indica cultivars (Ellis et al., 1992). Those investigations used seed lots produced at the International Rice Research Institute (IRRI) at Los Banos, Philippines (140 N). Whereas the indica and javanica rices evolved in climates roughly similar to those pertaining at Los Banos, the japonica rices evolved in somewhat cooler, more temperate environments (Purseglove, 1972; Yoshida, 1983; Chang, 1989). This led us to question whether the seed storage characteristics of japonica rices were intrinsically poorer than indica rices or whether the results described above 1993 Annals of Botany Company

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4 ~ 1.; J ; & I 586 Ellis et al.~seed Development, Maturation and Longevity of Rice ~ =0~=.~ ~ A -'-- c E hi"-,,'. '1:--t:..-8-,-",--A 8 ;:; ~ 1",8,-.,A B : 1 s!.--8' ~.;' ~ D Ii"- ;,,,.: )' i../t!,, :',.,'-.",!.~ :'..-',, ~' I I ' i.',',../,..i I,.-- " "'6 8--' '.A' I" f s, 1 s ~. ~ ,8- ' ,6' /,.- /J8 1,. ~6;' " i, 8'! ',: 8" ' '" ; 8 ; d ' i 16--A--6 i I',,! 'i.., '6--6.t s F -8:::'::--8--I:-~.--I,-...-t ,,-W--;" ~ /' /'~,,'-. A/,A/" ~ I,,., 8"'&"-' '~ -L-- 0 ~V ; : I lu ~O Time from 50% anthems (d) FIG. 2. Changes in ability to germinate normally in standard tests at 34/11 C for freshly-harvested (8) or dried (.) seeds ofthejaponica rice cv. Taipei 309 (A, B), the non-glutinous indica rice cv. Intan (C, D), and the glutinous indica rice cv. KetanAmnera (E, F) harvested serially during seed development and maturation from plants grown in controlled environments of 28/20 C (A, C, E) or 32/24 C (B, D, F). The triangles show the interim results for dried seeds after 28 d in test but before the removal of the seed covering structures. Hence the difference between squares and triangles is indicative of the degree of dormancy. The arrows indicate mass maturity; S indicates when seeds began to be shed from the plants. t s sealed in separate laminated aluminium foil bags. These were then stored on the same shelf (within each cultivar) of an incubator maintained at 40 C (::to'5 C). Samples were removed for germination tests at regular intervals for up to 39 d for the japonica seed lots and for up to 49 d for the indica seed lots. The longevity of the three seed lots provided by IRRI (Table 1) was also determined at 40 C with 15 % moisture content. Given that, within each tunnel, the three cultivars had been grown as three separate seed 'crops', and that the subsequent laboratory work was staggered slightly, the results for seed survival during storage were analysed separately for each cultivar. Seed survival curves were fitted to the data in accordance with eqn (1) by pro bit analysis tions shown by solid symbols in Fig. 1. The estimates of the time of mass maturity varied only between 17.9 and 20.4 d from 50 % anthesis (Table 2). These durations were always shorter in the warmer regime, but final mean seed dry weights were unaffected (cvs Taipei 309 and Intan) or only slightly reduced (cv. Ketan Amnera) in the warmer regime because seed-filling rates were greater (Table 2). The seeds produced in controlled environments were consistently lighter than those produced in the field at IRRI (compare Tables 1 and 2). Seed moisture contents at mass maturity varied between 36 and 43% (mean 40%) (Fig. 1). In contrast to the consistent, but slow, reduction in seed moisture content after mass maturity in cvs Taipei 309 (Fig. 1 A) and Intan using GLIM, in which analyses the criterion of survival (Fig. 1 B), the moisture content of the maturing seeds of the was the ability of a seed to produce a normal seedling (International Seed Testing Association, 1985a, b). glutinous indica cv. Ketan Amnera remained stable at about % moisture content between 27 and 47 dafter 50 % anthesis (Fig. 1 C). RESULTS Ability to germinate increased rapidly between the first and fourth harvests, with comparatively few (typically no Plants of cv. Intan reached 50 % anthesis soonest in both more than 20 %) of the seeds capable of germinating environments, and progress to flowering was more rapid in normally being damaged by desiccation to % moisture the warmer than the cooler regime in all three cultivars content (Fig. 2). Dormancy (compare triangles and squares (Table 2). However, in cv. Taipei 309 this difference in in Fig. 2) was often considerable in early harvests, was duration was negligible (thereby indicating that the optimum reduced during seed development and maturation, was temperature for rate of floral development is coolest in thiscultivar). reduced in the warmer compared to the cooler seed The durations from 50 % anthesis to mass maturitywere production regime, and was least in the japonica cultivar. determined by an objective iterative regression analysisprocedure The majority of the seeds harvested 2-4 d after mass (Pieta Filho and Ellis, 1991 a) with the observa- maturity (i.e., 22 dafter 50% anthesis) were capable of

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7 Ellis et at. DISCUSSION There are theoretical reasons for arguing that potential longevity (quantified here by the value of the seed lot constant, Kj, of the seed viability equation) is a good indicator of seed quality and vigour in many crops (Ellis~nd Roberts, 1980b, Moreover, in long-term seed stores for plant genetic resources conservation (i.e., seed genebanks) potential longevity is the criterion of seed quality of direct concern. The results presented here for the changes during seed development and maturation in the potential longevity of seeds of the two indica rice cultivars produced in both temperature regimes and for those of the japonica rice cultivar produced in the cooler regime (Fig. 4) are in general agreement with the results of earlier studies in other crops (Kameswara Rao et al., 1991; Pieta Filho and Ellis, 1991a; Demir and Ellis, 1992a, b, 1993; Ellis and Pieta Filho, 1992; Zanakis et al., 1994): maximum potential longevity was not attained until some time (12-19 d) after mass maturity. Hence, these results contradict the hypothesis (Harrington, 1972) that maximum seed quality is attained at the end of the seed~filling phase and that viability and vigour then decline. In contrast, the results for the japonica cultivar produced in the warmer regime do satisfy the first part of the above hypothesis, although the reduction in Kj values consequent upon delaying seed harvests for 15 d beyond mass maturity was negligible (squares in Fig. 4A) and the germination capacity of freshly-harvested seeds showed no decline at all in the 33 d after mass maturity (circles in Fig. 2B). It has long been recognised that high seed production temperatures can be deleterious to seed quality; for example, this has been documented in soyabean [Glycine max (L.) Merrill] (Green et al., 1965; TeKrony et al., 1980). At first sight, the effect of the temperature of the seed production regime on the changes in rice seed quality during development and maturation appear inconsistent between the japonica cultivar and the two indica cultivars (Fig. 4). However, in all three cultivars it can be seen that a deleterious effect of the higher temperature seed production environment on seed quality is detectable, but not until after mass maturity. In particular, the increase in Kj values subsequent to 22 dafter 50 % anthesis (the first seed harvest after mass maturity) is similar for the two indica cultivars and very much less for seeds produced in the warmer than the cooler regime, viz 0,4 (cv. Intan), 0,6 (cv. Ketan Amnera), 2.2 (cv. Intan) and 2.3 (cv. Ketan Amnera) respectively (Fig. 4B, C). This observation was tested by analysing the trend of the estimates of Kj with time from mass maturity. These analyses showed that, between 22 and 37- dafter 50 % anthesis, ~ increased significantly (P < 0'005) with a significant difference between the two temperature regimes in the rate of change (P < 0'005), but with no difference in this rate between the two indica cultivars within each regime (P> 0'10); the increase was only d-l (s.e. 0'0097) in the warmer regime, but 0'154 d-1 (s.e. 0'0105) in the cooler regime (i.e., six times greater, or an absolute difference of d-l). In this sense then, the difference between the two temperature regimes had a roughly consistent effect on the.seed Development, Alaturation and Longevity of Rice 589 change in K, values with time from mass maturity in all three cultivars; however, because the rate of increase in the japonica cultivar in the cooler regime (0'077 d-1, s.e. 0'0270) was only half that in the two indica cultivars, the effect of the increase in the temperature of the seed production regime for the japonica cultivar was to result in a negative trend of K, values after mass maturity (-0'012 d-1, s.e. 0'0018). Indeed, in this cultivar the effect of the increase in seed production environment temperature reduced the post mass maturity trend in K, values by only 0,089 d-1; in this sense, it could be argued that seed quality in cv. Taipei 309 (though poorer overall) was in fact slightly less sensitive to the increase in seed production temperature imposed here than the two indica cultivars. The demonstration here that this effect of increase in temperature is sufficient in the japonica rice cv. Taipei 309 to influence the stage during seed development and maturation when seeds attain maximum quality (Fig. 4A) may provide an explanation for the contradictory conclusions reached between certain seed quality development studies in other crops. This is particularly so in soyabean in which crop research in the USA has provided results which supported the Harrington hypothesis (TeKrony et al., 1980), whereas UK research (probably at cooler temperatures) showed that maximum potential longevity was not attained until some time after mass maturity (Zanakis et al., 1994). Whether or not the above speculation is valid remains to be seen. However, it is clear that a hot seed production environment can bring forward to an earlier developmental stage the time at which maximum seed quality is attained (Fig. 4A), or (in less vulnerable cultivars) it reduces the improvement in seed quality that occurs subsequent to mass maturity (Fig. 4 B, C). This demonstration provides an opportunity to researchers investigating the biochemical basis of seed quality to break the association between seed development, maturation and seed quality. The general comments of Chang (1991) thatjaponica rice cultivars show poorer longevity than indica rice cultivars are supported by these results. Even in the cooler seed production regime, the maximum potential longevity of the japonica cultivar was slightly less than that of the two indica cultivars, while the estimate of 0' was also marginally briefer (Fig. 4). Nevertheless, the magnitude of the difference in longevity betweenjaponica and indica cultivars reported by Chang (1991) may have resulted from the natural seed production environment at Los Banos being too warm for the japonica accessions as compared with the indica (and javanica) rice accessions. The gene bank at IRRI currently holds more than accessions-of which about 8 % are japonica rices. At present, but only when necessary, all accessions are regenerated in the field at IRRI. Now, although the research reported here was limited to a single japonica cultivar, it implies that the seed production environment at IRRI may be too harsh for japonica cultivars and that, in so far as seed accessions for the IRRI genebank are concerned, japonica accessions might be better regenerated in a cooler environment. Such seed is likely to remain viable more than three times as long as that produced at Los Banos (assuming a regeneration standard of 85 %). Clearly, further research

8 590 'i Ellis et al.-seed Development, A1 aturation and Longevity of Rice with a wide range of japonica accessions is required to confirm this implication. Since upland javanica cultivars belong to the same isozyme polymorphism group (VI) as the classicaljaponica rices (Glaszmann, 1986), it would also be helpful to include upland javanica rice cultivars i~ further research. Finally, the results presented here provide additional support to the conclusion of Roberts (1963), from an elegant investigation in six cultivars of rice, that while dormancy and longevity may often be positively correlated the relation is not causal. For example, comparison between Figs 2C and 4B shows that from 22 to 37 dafter 50% anthesis dormancy was reduced whereas potential longevity increased for cv. Intan in the cooler regime and, moreover, that from 37 to 52 dafter 50 % anthesis the decline in dormancy continued whereas no further change in potential longevity occurred. ACKNOWLEDGEMENTS This research was funded by the Holdback Facility of the UK Overseas Development Administration via the International Rice Research Institute. The engineering assistance of Messrs K. Chivers and S. Gill and Dr G. N. Zanakis of the Plant Environment Laboratory, Department of Agriculture, University of Reading is gratefully acknowledged. LITERATURE CITED Chang Yr Domestication and spread of the cultivated rices. In: Harris DR, Hillman GC, eds. Foraging andfarming: The evolution of plant exploitation. London: Unwin Hyman, Chang Yr Findings from a 28-yr seed viability experiment. International Rice Research Newsletter 16: 5-6. Collinson ST, Ellis RH, Summerfield RJ, Roberts EH Durations of the photoperiod-sensitive and photoperiod-insensitive phases of development to flowering in four cultivars of rice (Oryza sativa L.). Annals of Botany 70: Demir I, Ellis RH. 1992a. Changes in seed quality during seed development and maturation in tomato. Seed Science Research 2: Demir I, Ellis RH. 1992b. Development of pepper (Capsicum annuum) seed quality. Annals of Applied Biology 121: Demir I, Ellis RH Changes in potential seed longevity and seedling growth during seed development and maturation in marrow. Seed Science Research (in press). Ellis RH, Hong TD, Roberts EH Procedures for the safe removal of dormancy from rice seed. Seed Science and Technology 11: Ellis RH, Hong TD, Roberts EH The low-moisture-content limit to the negative logarithmic relation between seed longevity and moisture content in three subspecies of rice. Annals of Botany 69: Ellis RH, Pieta Filho C Seed development and cereal seed longevity. Seed Science Research 2: 9-~5. Ellis RH, Roberts EH. 1980a. Improved equations for the prediction of seed longevity. Annals of Botany 45: Ellis RH, Roberts EH. 1980b. Towards a rational basis for testing seed quality. In: Hebblethwaite PD, ed. Seed production. London: Butterworths, Ellis RH, Roberts EH The quantification of ageing and survival in orthodox seeds. Seed Science and Technology 9: Glaszmann JC A varietal classification of Asian cultivated rice (Oryza sativa L.) based on isozyme polymorphism. In: Rice Genetics. Los Banos, The Philippines: IRRI, Green DE, Pinnell EL, Cavanah LE, Williams LF Effect of planting date and maturity date on soybean seed quality. Agronomy Journal 57: Harrington JF Seed storage longevity. In: Kozlowski IT, ed. Seed biology, volume /I/. New York: Academic Press, International Seed Testing Association. 1985a. International rules for seed testing. Rules Seed Science and Technology 13: International Seed Testing Association. 1985b. International rules for seed testing. Annexes Seed Science and Technology 13: Kameswara Rao N, Appa Rao S, Mengesha MH, Ellis RH Longevity of pearl millet (Pennisetum glaucum R.Br.) seeds harvested at different stages of maturity. Annals of Applied Biology 119: Pieta Filho C, Ellis RH a. The development of seed quality in spring barley in four environments. I. Germination and longevity. Seed Science Research 1: Pieta Filho C, Ellis RH. 1991b. The development of seed quality in spring barley, in four environments. II. Field emergence and seedling size. Seed Science Research 1: Purseglove JW Tropical crops. Monocotyledons. London: Longman. Rasyad DA, Van Sanford DA, TeKrony DM Changes in seed viability and vigour during wheat seed maturation. Seed Science and Technology 18: Roberts EH An investigation of inter.varietal differences in dormancy and viability of rice seed. Annals of Botany 27: Summerfield RJ, Collinson ST, Ellis RH, Roberts EH, Penning de Vries FWT Photothermal responses of flowering in rice (Oryza sativa). Annals of Botany 69: TeKrony DM, Egli DB, Phillips AD Effects offield weathering on the viability and vigor of soybean seed. Agronomy Journal 72: Yoshida S Rice. In: Smith WH, Banta SJ, eds. Potential productivity offield crops under different environments. Los Banos, The Philippines: IRRI. Zanakis GN, Ellis RH, Summerfield RJ Seed quality in relation to seed development and maturation in three genotypes of soyabean (Glycine max). Experimental Agriculture (in press).